They had to threaten him to get him out

It worked

He wasn’t threatened with 25th amendment. That process takes too long. The Cabinet must convene a meeting and take a vote . The President needs to be provided in writing with their demand for him to step down. The President has 30 days to respond in writing if he chooses not to resign. The cabinet then notifies the Speaker of the House. Hearings must be held by the House in regards. Then the House must vote to approve or deny the Cabinets demand.

Good luck with all that happening in time to put another name on the ballot.

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They threatened to remove him from office , he chose to take himself off the ballot in exchange for finishing his term as President

Pulling the 25th on Biden would have been a big embarrassing pill for the Dems to swallow.